1. Field of the Invention
The present invention relates to a position measuring device, and an error detecting method for the device, and more particularly to reduction of the power consumption of a position measuring device which detects displacement between two members.
The present invention relates also to an electromagnetic induction position detecting device which detects a relative position of two members with using electromagnetic coupling between the members, and more particularly to a method of detecting a fault such as a breakage of a coil.
2. Description of the Related Art
Measurement instruments such as an electronic caliper are widely used for measuring the thickness or other physical dimensions of an object in the manufacturing industry. A transducer is used as the principal component of such a measurement instrument.
Known transducers include a capacitive transducer and an inductive transducer. In the capacitive transducer, a transmitter electrode and a receiver electrode are disposed on a grid (slider), and signal electrode is disposed on a scale which is opposed to the grid. The transmitter and receiver electrodes on the grid are capacitively coupled with the signal electrode on the scale. A driving signal is supplied to the transmitter electrode, and a detection signal, which appears in the receiver electrode in accordance with the relative position between the grid and the scale, is processed by a signal processing circuit, thereby detecting the movement or position of the grid with respect to the scale.
On the other hand, in the inductive transducer, the relative position is detected on the basis of electromagnetic induction between a grid and a scale. For example, an exciting coil is disposed in the grid to generate a magnetic flux, thereby causing an induced current to be generated in a scale coil. The induced current generates a magnetic flux, so that an induced current (induced voltage) is generated in a detection coil disposed in the grid. The induced voltage generated in the detection coil is changed in accordance with relative displacement between the grid and the scale, and hence the displacement can be detected based on an induced-voltage signal.
In a measurement instrument of the related art, in order to ensure that the transducer surely operates, a CPU monitors a signal from the transducer at each sampling timing to judge whether the transducer is normal or not.
In the configuration in which the CPU is operated at each sampling timing to perform error detection for judging whether the transducer is normal or not, however, the power consumption is increased. When a battery is used as the power source, for example, there arises a problem in that the lifetime of the battery is shortened.
In the case where an inductive transducer is used, particularly, there is an advantage that the transducer is operable even in an environment of higher pollution as compared with the case where a capacitive transducer is used. However, the inductive transducer has a problem that the power consumption is large. Therefore, the above-mentioned increase of the power consumption: is not preferable.
In an electromagnetic induction position detecting device of the related art which uses electromagnetic coupling between a detection head (first member) and a scale (second member), the relative position of the scale with respect to the detection head is detected on the basis of electromagnetic induction between the detection head and the scale. More specifically, the electromagnetic induction position detecting device is configured by: the detection head in which a driving coil and a receiving coil are disposed; and the scale relatively moved with respect to the detection head in which magnetic modulating means such as closed loop coils is disposed. The magnetic modulating means modulates the magnetic flux generated by the driving coil. A primary changing magnetic flux which is generated when the driving coil is AC-driven is modulated by the scale, so that a magnetic pattern of a predetermined period is formed. The magnetic pattern is coupled with the receiving coil of the detection head. As a result, an induced voltage which is varied in accordance with movement of the scale appears in the receiving coil. When variations of the induced voltage are detected, it is possible to detect the relative movement distance of the scale with respect to the detection head. Alternatively, an absolute position detection is enabled in such a manner that a plurality of position detection tracks, in which magnetic patterns are formed in different periods, are disposed, the induced voltages are respectively detected by receiving coils of the tracks, and a phase difference between the induced voltages of the tracks is detected.
However, in the electromagnetic induction position detecting device, malfunction caused by coil breakage cannot be detected, and there is no means for, when the position is erroneously detected, informing the operator of the erroneous detection. Furthermore, it is difficult to stably and surely detect malfunction caused by breakage of the scale coil because a change due to such malfunction is very small in level.
It is a first object of the invention to provide a position measuring device and an error detecting method for the device, in which an error of a transducer can be detected at a low power consumption.
It is a second object of the invention to provide an electromagnetic induction position detecting device in which occurrence of a fault such as a breakage of a coil can be detected stably and surely, thereby enabling correct position detection to be performed.
In order to attain the first object, the position measuring device for detecting displacement between two members, according to the invention, comprises: a transducer which outputs an electric signal based on displacement between the two members; and a detecting section which, when a relative speed between the two members is equal to or smaller than a predetermined value, performs detection of malfunction of the transducer. In the device, detection of malfunction (error detection) of the transducer is performed not at a predetermined time interval, but at a timing when the relative speed is equal to or smaller than the predetermined value. Therefore, power consumption due to the detection of malfunction can be reduced.
Preferably, the detection of malfunction may be performed when the relative speed between the two members is zero. In the case where the relative speed is zero, it is particularly necessary to accurately detect the position. When the detection of malfunction of the transducer is performed at that position, therefore, the detection of malfunction can be efficiently performed, and the accuracy of the detection of position can be ensured.
For example, the transducer may be an inductive transducer. An inductive transducer itself consumes a relatively large power. When the power consumption due to the detection of malfunction is reduced, therefore, the ease of use of the inductive transducer is improved. In the case where the transducer is powered by a battery, the lifetime of the battery can be prolonged.
Alternatively, the transducer may be a transducer of an absolute, type electronic caliper which detects displacement from a predetermined reference position, i.e., the absolute position. In an incremental type electronic caliper, the displacement amount is sequentially detected. By contrast, in the absolute type electronic caliper, it is often that the absolute position of a grid is measured while the grid is made still with respect to a scale. When the detection of malfunction of the transducer is performed in the case where the relative speed of the grid is equal to or smaller than a predetermined value, or where the relative speed is zero, therefore, the timing of the position detection can be synchronized with that of the detection of malfunction, and hence the process can be efficiently conducted.
Preferably, the detecting section may detect the malfunction by using a ratio of an amplitude of the electric signal for detecting the displacement, to an amplitude in a normal condition. When the transducer itself becomes faulty, when the distance between the scale and the grid is changed, or when a foreign matter enters the transducer, the amplitude of the detection signal is changed from that in a normal condition. By using a ratio of the amplitude of the detection signal to that in a normal condition, therefore, malfunction of the transducer can be surely detected.
In order to attain the second object, a first electromagnetic induction position detecting device according to the invention comprises: a first member; a second member which is disposed to be opposed to the first member with a predetermined gap in a relatively movable manner; a driving coil which is disposed on the first member, and which generates a primary changing magnetic flux; a magnetic modulating section which is disposed on the second member, and which modulates the primary changing magnetic flux to generate a magnetic pattern that is changed in a predetermined spatial period and in a direction of the relative movement; polyphase receiving coil which is disposed on the first member to detect the magnetic pattern, and which is formed in the predetermined spatial period and with portions of the polyphase receiving coil being shifted in spatial phase from one another; a transmission signal generating section which supplies a transmission signal for generating the primary changing magnetic flux, to the driving coil; a position calculating section which performs a calculating process on polyphase reception signals that are received by the receiving coils, to detect a relative position of the second member with respect to the first member; and a fault detecting and informing section which detects occurrence of a fault from amplitude balance among the polyphase reception signals, and which informs of the occurrence.
According to the invention, it is noted that, when a fault such as a breakage of a coil occurs, the amplitude balance among the polyphase reception signals is lost, and therefore the fault is detected on the basis of the amplitude balance, and the fault occurrence is informed. Therefore, the operator is enabled by a very simple calculation to be informed of the occurrence of a fault. As a result, measurement is prevented from being conducted under an abnormal condition, so that correct measurement can be always performed.
A second electromagnetic induction position detecting device according to the invention comprises: a first member; a second member which is disposed to be opposed to the first member with a predetermined gap in a relatively movable manner; a plurality of position detection tracks of different spatial periods which are formed on the first and second members. Each of the tracks comprising: a driving coil which is disposed on the first member, and which generates a primary changing magnetic flux; a magnetic modulating section which is disposed on the second member, and which modulates the primary changing magnetic flux to generate a magnetic pattern that is changed in a spatial period inherent to the track and in a direction of the relative movement; and polyphase receiving coil which is disposed on the first member, which detects the magnetic pattern, and which is formed in the spatial period and with portions of the polyphase receiving coil being shifted in spatial phase from one another. The electromagnetic induction position detecting device also comprises: transmission signal generating section which supplies a transmission signal for generating the primary changing magnetic flux, to the driving coil of each of the tracks; a position calculating section which performs a calculating process on polyphase reception signals output from the receiving coil of each of the tracks, to detect an absolute position of the second member with respect to the first member; and a fault detecting and informing section which detects occurrence of a fault from amplitude balance among the polyphase reception signals of the tracks and which informs of the occurrence.
According to the invention, a loss of the amplitude balance of the polyphase reception signals which is caused among tracks by a fault such as a breakage of a coil is detected, and the fault occurrence is informed. Therefore, the operator is enabled by a very simple calculation to be informed of the occurrence of the fault. As a result, measurement is prevented from being conducted under an abnormal condition, so that correct measurement can be always performed. In the invention, moreover, even when the gap between the first and second members is varied, the change of the amplitude level appears in the same manner among the tracks. Therefore, the detection accuracy is not affected by such variation.
For example, the amplitude balance may be detected also from the sum of amplitudes of the polyphase reception signals. When the amplitude balance is detected from the sum of the squares of the amplitudes, i.e., the signal level, it is possible to detect a minute change of the accuracy with higher sensitivity. For example, the amplitude balance among the tracks may be monitored based on the difference or a ratio of the signal levels among the tracks. Alternatively, when a certain event occurs, the signal levels may be stored, and the levels of the polyphase reception signals of the tracks may be normalized by the stored signal levels. In the alternative, accurate fault detection can be always performed without being affected by dispersion of, or large difference between, signal levels among products.